The present invention relates to a method of making a plastic cap, and more particularly to a method of manufacturing, by means of compression molding, a plastic cap with a threaded inner surface employed for hermetically sealing the mouth of a container such as a bottle.
A typical conventional plastic cap, i.e., a plastic cap with a threaded inner surface employed for hermetically sealing the mouth of a container such as a bottle made of glass, plastic or metal is manufactured by mainly injection molding. The injection molding is a method wherein a molten plastic is injected from a nozzle into a mold to form a molded article with a predetermined shape. Since the method requires the material to have an excellent fluidity, it is general to employ a relatively low-strength plastic having a relatively low molecular weight and a melt flow index of about 10-20 g/10 min.
Accordingly, in order to ensure a required cap strength, it is necessary to make the top end and skirt portion of the cap have a relatively large wall thickness, which unfavorably results in a higher material cost.
Moreover, as a plastic stock, it is preferable from the viewpoint of colt to select a material from polyolefins, e.g., polyethylene or polypropylene. However, caps made of these olefin resins by means of injection molding are susceptible to stress-crack, and are poor in low-temperature impact strength. Further, since this type of caps are also poor in the barrier properties to O.sub.2 and CO.sub.2, products contained in the bottles hermetically sealed by the caps are inconveniently apt to change properties thereof.
On the other hand, a method of manufacturing plastic caps by means of compression molding has recently been proposed in Japanese Patent Laid-Open No. 501121/1981 (PCT/US80/01142) or the like. The method is such that a molten plastic stock is loaded in a cavity with a shape corresponding to the outer shape of a cap to be formed and then a plunger having in its side surface a groove with a shape corresponding to that of a thread to be formed in the inner surface of the cap is forced into the cavity to compression-mold a cap with a threaded inner surface.
In this method, however, since the end of the skirt portion of the cap is formed in the last stage, in order to ensure an accurate profile of the end portion, it is necessary to employ a plastic excellent in fluidity, i.e., relatively high in melt flow index (although not so high as is in the injection molding).
In addition, this method unfavorably requires a comparatively high molding pressure. Particularly, in case of manufacturing a cap having a pilfer-proof band connecting with the skirt portion through narrow bridges, the flow of a resin is apt to clog (owing to setting) at the bridges. Consequently, a molding fault, such as a missing portion, is easily produced at the pilfer-proof band, and moreover, it is inconveniently difficult to form a pilfer-proof band with a complicated shape. Particularly, in case where the skirt portion is high, these shortcomings tend to occur remarkably.
In addition, since the cavity is deep, it is often that when the molten plastic stock is loaded in the cavity by dropping, the plastic stock is caught by the sidewall of the cavity, resulting in an unsatisfactory compression molding. Moreover, since the amount of air to be removed in the cavity is large, it is inconveniently necessary to employ a special means for removal of the air.